Underground water pumping device

ABSTRACT

A groundwater pumping apparatus includes an outer tube, an inner tube, a shield member, a vacuum unit and a pump unit. The outer tube is buried in ground, and has a water-passing portion at its lower part. The inner tube has a substantially same length as the outer tube, and is placed in the outer tube such that the inner tube is spaced from the outer tube. The inner tube has a groundwater inlet at its bottom end, which is provided at a position lower than a top end of the water-passing portion. The shield member shields top ends of the outer tube and the inner tube. The vacuum unit reduces a pressure in the inner tube. The pump unit pumps groundwater that comes in the inner tube through the groundwater inlet.

TECHNICAL FIELD

The present invention relates to a groundwater pumping apparatus forpumping groundwater to lower the groundwater level in the case ofexcavating ground at underground construction sites or performing groundimprovement.

BACKGROUND ART

In the past, when excavating ground at underground construction sites orperforming ground improvement, groundwater is pumped to lower thegroundwater level. Thus, by pumping groundwater to lower the groundwaterlevel, it is possible to reduce a water content in soils of ground at adesired location. As a result, the excavating operation or treatmentsfor the excavated soils can be facilitated.

As a groundwater pumping apparatus, a deep-well apparatus is known fromthe past. For example, in a conventional deep-well apparatus shown inFIG. 11, a well 40 having a water-passing portion 41 (strainer), throughwhich groundwater comes inside, is put in a boring 43 formed in ground42. In addition, a pump 46 is placed in the well 40. A clearance betweenan inner wall of the boring 43 and an outer surface of the well 40 isfilled with a filter material 45 such as coarse sand or pea gravel.Groundwater is drawn from the water-passing portion 41 in the well 40 bythe aid of a hydraulic head drop between a water level in the well 40and the natural water level, and then the collected groundwater ispumped by the pump 46.

However, since the hydraulic head drop between the water level in thewell 40 and the natural water level is used to collect groundwater, thegroundwater collecting capacity depends on gravity difference. For thisreason, groundwater can not be collected under a condition ofeffectively lowering the groundwater level.

In addition, a vacuum deep-well apparatus is known, which has thecapability of effectively lowering the groundwater level by placing avacuum unit in the deep-well apparatus, and reducing a pressure in thewell, is known. However, in this vacuum deep-well apparatus, there is aproblem that when the groundwater level reaches a position lower thanthe top end of the water-passing portion, air is drawn together withgroundwater in the well through the water-passing portion, so that thevacuum effect of the vacuum unit sharply deteriorates. Thus, it stillhas plenty of room for improvement from the point of stably providingthe groundwater collecting/pumping operation.

On the other hand, as shown in FIG. 12, a groundwater pumping apparatusdisclosed in Japanese Patent Early Publication No. 2000-27170 has astrainer device 48 at a lower end of a casing tube 47 buried in ground42. This strainer device 48 is formed with a strainer tube 49 attachedto the lower end of the casing tube 47, sand accumulator 50 formed at alower end of the strainer tube 49, and an inner tube 51 attached to thestrainer tube 49 in a concentric manner with the casing tube 47. Thestrainer tube 49 is attached such that an outer surface of the casingtube 47 is substantially flush with the outer surface of the strainertube 49. A clearance 52 is made between the strainer tube 49 and theinner tube 51, and closed at its upper end by a blockage plate 53 placedat the lower end of the casing tube 47. A groundwater inlet 54 is formedin the inner tube 51 at a position lower than the top end of awater-passing portion 55 of the strainer tube 49. In addition, a pump 56for pumping groundwater is placed in the inner tube 51. The top end ofthe casing tube 47 is closed by a cover 58. In FIG. 12, the numeral 57designates a vacuum unit for reducing a pressure in the casing tube 47connecting to the inner tube 51.

In the case of using this pumping apparatus, when the pressure in thecasing tube 47 connecting to the inner tube 51 is reduced by use of thevacuum unit and the hydraulic head drop, groundwater is drawn from thewater-passing portion 55 of the strainer tube 49 in the inner tube 51through the clearance 52 and the groundwater inlet 54, and collected inthe inner tube 51. The collected groundwater in the inner tube 51 ispumped by the pump 56. Even when the groundwater level is lower than thetop end of the water-passing portion 55, no intrusion of air into theinner tube 51 is caused under a condition that the groundwater level ishigher than the position of the groundwater inlet 54. Therefore, it ispossible to prevent the deterioration of the vacuum effect by the airintrusion, which occurs in the vacuum deep-well apparatus describedabove.

However, in this groundwater pumping apparatus, it is necessary to placethe strainer device at the lower end of the casing tube 47. Therefore,when a deep-well apparatus having poor groundwater collecting capabilityhas already existed in ground 1, the groundwater collecting/pumpingoperation by use of the already-existing apparatus is stopped, and thepumping apparatus of JP 2000-27170 must be newly formed in ground 1 torestart the water collecting/pumping operation. Thus, there is a problemthat the existing deep-well apparatus comes to naught, and constructionsfor a groundwater pumping apparatus having another structure becomenecessary.

Moreover, since the groundwater pumping apparatus of JP 2000-27170 hasthe structure that the strainer device 48 composed of the inner tube 51,the clearance 53 and the strainer tube 49 is placed under the casingtube 47, groundwater can be collected only through the strainer device48 placed under the casing tube 47. Therefore, when groundwater ispresent in the vicinity of an upper part of the casing tube 47 due tostratum structure, it may not be effectively collected. In addition, inthis pumping apparatus, when the groundwater level lowers to reach thegroundwater inlet 54 of the inner tube 51, air is sharply drawn in theinner tube 51 in place of groundwater, so that the vacuum effect sharplydeteriorates. For this reason, the pump 56 must be stopped until thegroundwater level raises again. Subsequently, when the groundwater levelis recovered, the pressure in the inner tube 51 is reduced to a requiredvalue by the vacuum unit 57 and the groundwater collecting/pumpingoperation is restarted by use of the pump 56. Thus, there is a case thata stable groundwater collecting/pumping operation can not becontinuously carried out.

SUMMARY OF THE INVENTION

In view of the above problems, an object of the present invention is toprovide a groundwater pumping apparatus having the capability ofeffectively performing a groundwater collecting/pumping operation with arefined configuration, and making effective use of an existing deep-wellapparatus. Moreover, in the groundwater pumping apparatus of the presentinvention, even when groundwater is present in the vicinity of groundsurface, it is possible to effectively provide the stable groundwatercollecting/pumping operation. In addition, a passage for groundwater canbe maintained by effectively carrying out a water injection in ground.

That is, the groundwater pumping apparatus of the present inventioncomprises:

-   an outer tube 3 buried in ground 1, which has a water-passing    portion 2 at its lower part;-   an inner tube 5 having substantially a same length as the outer tube    and placed in the outer tube 3 so as to be spaced from the outer    tube, the inner tube 5 having a groundwater inlet 7 at its bottom    end, which is provided at a position lower than a top end of the    water-passing portion 2;-   a shield member 6 for shielding top ends of the outer tube 3 and the    inner tube 5;-   a vacuum means 8 for reducing a pressure in the inner tube; and-   a pump means 9 for pumping groundwater that comes in the inner tube    5 through the groundwater inlet 7.

When the pressure in the inner tube 5 is reduced by the vacuum means 8,groundwater comes in the clearance 4 from the water-passing portion 2 ofthe outer tube 3. Then, the groundwater flows into the inner tube 5 fromthe clearance 4 through the groundwater inlet 7 of the inner tube 5. Thegroundwater collected in the inner tube 5 is pumped by the pump means 9.In this case, when the groundwater level is lower than the top end ofthe water-passing portion 2, air comes into the clearance 4 through thewater-passing portion 2. However, since the groundwater inlet 7 isplaced at the position lower than the top end of the water-passingportion, the air that comes in the clearance 4 is collected in an upperregion of the clearance, but can not come in the inner tube 5.Therefore, it is possible to prevent a deterioration of vacuum effectsof the vacuum means 8, and efficiently carry out the groundwatercollecting/pumping operation.

In addition, even when wells having various diameters such as wellsformed by a deep-well method or a vacuum deep-well method, test well forinvestigating ground, and a recharge well, has already existed, it ispossible to make effective use of these existing wells as the outer tube3 because the outer tube 3 has substantially the same length as theinner tube 5. That is, the already-existing well can be recycled for thegroundwater pumping apparatus of the present invention by inserting theinner tube 5 having substantially the same length as the outer tube 3 soas to make the clearance 4 between the inner tube and thealready-existing well as the outer tube 3, shielding the top ends of theouter tube 3 and the inner tube 5 with the shield member 6, and placingthe vacuum means 8 and the pump means 9.

In the groundwater pumping apparatus described above, it is preferredthat the outer tube 3 has a plurality of water-passing portions formedin an axial direction of the outer tube. In this case, the efficiency ofcollecting groundwater can be further improved.

Additionally, in the groundwater pumping apparatus described above, itis preferred that the groundwater inlet 7 is formed in a bottom endsurface of the inner tube 5 and placed at a position slightly higherthan the bottom end of the outer tube 3, and the inner tube 5 has an airinlet 11 composed of at least one slit 22 for air vent having anelongate shape, and the slit is formed in a side surface of the bottomend of the inner tube so as to extend in an axial direction of the innertube, and the air inlet 11 is formed to extend from the groundwaterinlet to a position lower than the top end of said water-passing portion2. During the pumping operation, a small amount of air comes in theinner tube 5 from the air inlet 11 before the groundwater level lowersto reach a level of the groundwater inlet 7, so that an internalpressure of the inner tube 5 slightly raises. By controlling the pumpingamount by the pump means 9 in response to the pressure change in theinner tube 5, it is possible to prevent an abrupt decrease ofgroundwater, and avoid a situation that groundwater can not be pumpedbecause a large amount of air suddenly flows in the inner tube from thegroundwater inlet 7. In addition, since the air inlet 11 is provided bythe elongate slit 22 extending in the axial direction of the inner tube5, the amount of air flowing from the slit 22 into the inner tube 5increases as the groundwater level lowers. Therefore, it is possible toaccurately adjust the pumping amount according to the pressure change inthe inner tube 5.

Moreover, in the groundwater pumping apparatus described above, it ispreferred that groundwater inlet 7 is formed in a bottom end surface ofthe inner tube 5 and placed at a position slightly higher than thebottom end of the outer tube, and the inner tube 5 has an air inletcomposed of a plurality of holes 23 for air vent, and the holes areformed in a side surface of the bottom end of the inner tube so as to bearranged in an axial direction of the inner tube, and the air inlet 11is formed to extend from the groundwater inlet 7 to a position lowerthan the top end of the water-passing portion 2. During the pumpingoperation, a small amount of air comes in the inner tube 5 from the airinlet 11 composed of the plurality of holes 23 for air vent before thegroundwater level lowers to reach a level of the groundwater inlet 7, sothat an internal pressure of the inner tube 5 slightly raises. Bycontrolling the pumping amount by the pump means 9 in response to thepressure change in the inner tube 5, it is possible to prevent an abruptdecrease of groundwater, and avoid a situation that groundwater can notbe pumped because a large amount of air suddenly flows in the inner tubefrom the groundwater inlet 7. In addition, since the air inlet 11 isprovided by the plurality of holes 23 for air vent formed in an axialdirection of the inner tube, the amount of air flowing from the holes 23for air vent into the inner tube 5 increases as the groundwater levellowers. Therefore, it is possible to accurately adjust the pumpingamount according to the pressure change in the inner tube 5.

In the above-described groundwater pumping apparatus, it is alsopreferred that an aperture amount of a lower part of the air inlet 11 islarger than the aperture amount of its upper part. In this case, as thegroundwater level lowers, the amount of air flowing into the inner tube5 increases in a quadratic-function manner. Therefore, it is possible toaccurately detect delicate fluctuations of the water level at the airinlet 11.

It is further preferred that the groundwater pumping apparatus describedabove comprises a pressure detecting means 12 for detecting the internalpressure of the inner tube, and a pumping-amount adjusting means 13 foradjusting a pumping amount. It is effective to provide the stablepumping operation without interruption.

It is also preferred that the groundwater pumping apparatus describedabove further comprises a pressure means 14 for increasing the pressurein the inner tube, and a water injection means 15 for carrying out apressure injection of water in the inner tube. In this case, it ispossible to clean clogging in a filter member 16 placed around the outertube 3 and the water-passing portion 2, and provide a more effectivegroundwater collecting/pumping operation.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic cross-sectional view of a groundwater pumpingapparatus according to a first embodiment of the present invention;

FIG. 2 is a schematic cross-sectional view of a groundwater pumpingapparatus according to a second embodiment of the present invention;

FIG. 3 is a schematic cross-sectional view of a groundwater pumpingapparatus according to a third embodiment of the present invention;

FIG. 4 is a schematic cross-sectional view of a groundwater pumpingapparatus according to a modification of the second embodiment;

FIG. 5 is a schematic cross-sectional view of a groundwater pumpingapparatus according to another modification of the second embodiment;

FIG. 6 is a schematic cross-sectional view of a groundwater pumpingapparatus according to a modification of the third embodiment;

FIG. 7 is a schematic cross-sectional view of a groundwater pumpingapparatus according to another modification of the third embodiment;

FIG. 8 is a schematic cross-sectional view of a groundwater pumpingapparatus according to a further modification of the second embodiment;

FIG. 9 is a schematic cross-sectional view of a groundwater pumpingapparatus according to a fourth embodiment of the present invention;

FIG. 10 is a schematic cross-sectional view of a groundwater pumpingapparatus according to a fifth embodiment of the present invention;

FIG. 11 is a schematic cross-sectional view of a conventionalgroundwater pumping apparatus; and

FIG. 12 is a schematic cross-sectional view of a groundwater pumpingapparatus disclosed in Japanese Patent Early Publication No. 2000-27170.

BEST MODE FOR CARRYING OUT THE INVENTION

The present invention is explained below in detail according toembodiments shown in the attached drawings.

As shown in FIG. 1, a groundwater pumping apparatus according to a firstembodiment of the present invention is mainly composed of an outer tube3 buried in ground 1, an inner tube 5 having substantially a same lengthas the outer tube 3 and inserted in the outer tube 3 through a clearance4, a shield member 6 for shielding top ends of the outer tube 3 and theinner tube 5, a vacuum unit 8 for reducing a pressure in the inner tube5, and a pump unit 9 for pumping groundwater collected in the inner tube5.

The outer tube 3 has a water-passing portion 2 at its lower part atleast. For example, a steel tube 29 having a strainer tube 30 at itslower end can be used as the outer tube 3. The strainer tube 30 is of acylindrical shape having the same diameter as the steel tube 29, and hasa structure that a steel wire is wound at a required interval to formcrevices that function as the water-passing portion 2. Alternatively, atube having a plurality of apertures that function as the water-passingportion 2 may be used as the strainer tube 30. That is, thewater-passing portion 2 has a structure that groundwater can flow insidethrough the crevices. In the figure, the numeral 17 designates a sandaccumulator placed at the bottom end of the strainer tube 30. The bottomof the sand accumulator 17 provides the bottom end 10 of the outer tube3.

The inner tube 5 is formed by a steel tube having substantially the samelength as the outer tube 3. The bottom end surface of the inner tube 5is entirely opened to function as a groundwater inlet 7. This inner tube5 is concentrically inserted in the outer tube 3 to be spaced from theouter tube 3 by the clearance 4. The groundwater inlet 7 of the innertube 5 is provided at a position slightly higher than the bottom end 10of the outer tube 3, and at the position lower than the top end of thewater-passing portion 2.

Groundwater pumping apparatuses of the present invention shown in FIGS.2 to 7 are characterized in that an air inlet 11 is formed in a sidesurface of the bottom end of the inner tube 5 at a position higher thanthe groundwater inlet 7. In the groundwater pumping apparatus of thesecond embodiment shown in FIG. 2, a plurality of slits 22 for air venteach having an elongate shape are formed in the side surface of thebottom end of the inner tube 5 to extend in the axial direction of theinner tube. The slits 22 have a length from the groundwater inlet 7 to aposition lower than the top end of the water-passing portion 2. Theslits 22 provide the air inlet 11. Moreover, in the pumping apparatus ofthe third embodiment shown in FIG. 3, a plurality of holes 23 for airvent are formed in the side surface of the bottom end of the inner tube,and arranged in the axial direction of the inner tube. This arrangementof the holes 23 has a length from the groundwater inlet 7 to a positionlower than the top end of the water-passing portion 2. The holes 23provide the air inlet 11.

In addition, modifications of the pumping apparatus of the secondembodiment are shown in FIGS. 4 and 5, and modifications of the pumpingapparatus of the third embodiment are shown in FIGS. 6 and 7. In each ofthese pumping apparatuses, it is shown that the air inlet 11 has alarger aperture area at its lower part than its upper part. That is, inthe groundwater pumping apparatus of FIG. 4, each of the slits 22 is ofa triangular shape that tapers down toward the upper part of the innertube. Additionally, in the groundwater pumping apparatus of FIG. 5, eachof the slits 22 is composed of an upper slit having a narrow width and alower slit having a wide width. On the other hand, in the groundwaterpumping apparatus of FIG. 6, a plurality of holes 23 for air vent areformed such that the number of the holes 23 for air vent increasestoward the bottom side of the inner tube. In addition, in thegroundwater pumping apparatus of FIG. 7, the holes 23 for air vent areformed such that diameters of the holes 23 for air vent increase towardthe bottom side of the inner tube. In the case that the aperture area ofthe lower part of the air inlet 11 is larger than the aperture area ofthe upper part thereof, an embodiment other than FIGS. 4 to 7 may beadopted to obtain the larger aperture area of the air inlet 11 at thelower part than the upper part.

The top ends of the outer tube 3 and the inner tube 5 are shielded bythe shield member 6 such as a cover. A pump 24 is disposed as the pumpunit 9 in the vicinity of the groundwater inlet 7 at the bottom end ofthe inner tube 5. A pump tube 25 connected to the pump 24 air-tightlypasses through the shield member 6 and extends outside. At the outsideof the shield member 6, a pumping-amount adjusting unit 13 including aflow-amount regulating valve is placed.

A ventilation slot 28 is formed in the shield member 6. A vacuum pump 26that is the vacuum unit 8 is connected to this ventilation slot 28through a joint pipe 27. By activating the vacuum pump 26, the internalpressure of the inner tube 5 can be reduced. In the figure, the numeral12 designates a pressure detecting unit such as a pressure meter fordetecting the pressure in the inner tube 5.

The groundwater pumping apparatus having the above configuration is usedto lower the groundwater level by pumping groundwater in the case ofexcavating ground 1 at underground construction sites, or performingground improvement. That is, a boring 18 having a larger diameter thanthe outer tube 3 is initially formed in ground 1 at a location intendedfor pumping groundwater. Then, the outer tube 3 of the groundwaterpumping apparatus is inserted into the boring 18. At this time, a filtermaterial 16 such as coarse sand or pea gravel is charged in a spacebetween an inner wall of the boring 18 and the peripheral surface of theouter tube 3. In addition, a sealing material 19 such as cement mortaror bentonite mortar is charged in a space between the inner wall of anupper part of the boring 18 and the peripheral surface of an upper partof the outer tube 3. In place of the sealing material 19, anothersealing means may be used. For example, a balloon may be put between theinner wall of the upper part of the boring 18 and the peripheral surfaceof the upper part of the outer tube 3, and inflated to sealtherebetween.

In the case of pumping groundwater, when the internal pressure of theinner tube 5 is reduced by the vacuum unit 8, groundwater flows in theclearance 4 through the water-passing portion 2 of the outer tube 3. Inaddition, the groundwater forcedly flows in the inner tube 5 from theclearance 4 through the groundwater inlet 7 formed in the bottom end ofthe inner tube 5. The collected groundwater in the inner tube 5 ispumped by activating the vacuum pump of the vacuum unit 9. In this case,when the groundwater level lowers to reach the top end of thewater-passing portion 2, air existing in ground 1 comes into theclearance 4 from the water-passing portion 2. However, since thegroundwater inlet 7 of the inner tube 5 is provided at a position lowerthan the top end of the water-passing portion 2, air is accumulated inan upper part of the clearance 4, so that the air can not comes in theinner tube 5. Therefore, only groundwater flows in the inner tube 5 fromthe groundwater inlet 7. At this time, a small amount of groundwater canflow in the inner tube 5 from the air inlet 11 provided at the positionlower than the top end of the water-passing portion 2. Thus, even whenthe groundwater level lowers to reach the top end of the water-passingportion 2, it is possible to prevent the inconvenience that vacuumeffects of the vacuum unit 8 deteriorates by a flow of air in the innertube 5.

In addition, when the groundwater level further lowers to reach the topend of the air inlet 11 provided at the position lower than the top endof the water-passing portion 2, part of the air that came in theclearance 4 from the water-passing portion 2 can flow in the inner tube5 from the upper part of the air inlet 11, but not flow in the innertube 5 from the groundwater inlet 7. In this case, though the smallamount of air flows in the inner tube 5 to slightly increase theinternal pressure of the inner tube 5, the vacuum effects of the vacuumunit 8 does not sharply deteriorate, and the flow of groundwater in theinner tube 5 from the groundwater inlet 7 can be stably maintained.Therefore, it is possible to stably continue the groundwater pumpingoperation by the pumping unit 9.

When the groundwater level reaches the air inlet 11, and the groundwaterpumping operation is continued by the pumping unit 9 at a pumping ratethat is the same as the pumping rate before the decrease of groundwaterlevel, the groundwater level sharply lowers, so that there is a fearthat the groundwater level lowers to reach the groundwater inlet 7 thatis the bottom opening of the inner tube 5. When the groundwater levelreaches the groundwater inlet 7, a large amount of air suddenly flows inthe inner tube 5 from the groundwater inlet 7 that has a much largeraperture area than the air inlet 11 to rapidly increase the internalpressure of the inner tube 5. This leads to a considerable decrease inthe flow amount of groundwater in the inner tube 5 due to adeterioration of the vacuum effects. In such a case, the pumpingoperation using the pumping unit 9 is stopped, and it is necessary towait a rise in the groundwater level. Then, the interior of the innertube 5 is maintained again under a reduced pressure by use of the vacuumunit 8, and the groundwater pumping operation is restarted by thepumping unit 9. Thus, there is a fear of causing an interruption of thepumping operation.

In the present invention, when the groundwater level lowers, and thesmall amount of air flows in the inner tube 5 from the air inlet 11 toincrease the internal pressure of the inner tube 5, the increase in theinternal pressure of the inner tube 5 is detected by the pressuredetecting unit 12. When the internal pressure of the inner tube 5reaches a predetermined value or more, the pumping amount is reduced bythe flow-amount regulating valve of the pumping-amount adjusting unit13. As a result, it is possible to continue the groundwater pumpingoperation by the pumping unit 9 without interruption. The pumping amountmay be controlled by checking a detected value by the pressure detectingunit 12 with the naked eye, and adjusting the flow-amount regulatingvalve by manual operation. Alternatively, as shown in the groundwaterpumping apparatus shown in FIG. 8, a signal detected by the pressuredetecting unit 12 is output to a control unit 20, and the flow-amountregulating valve may be controlled according to a control signal fromthe control unit 20 to adjust the pumping amount.

As described above, when the pumping amount is controlled by thepumping-amount adjusting unit 13 according to the pressure change in theinner tube 5 detected by the pressure detecting unit 12, and the airinlet 11 is formed with the plurality of slits 22 or holes 23 for airvent, as shown in FIGS. 2 to 7, the amount of air flowing in the innertube 5 through the air inlet 11 increases as the groundwater levellowers. The adjustment of the pumping amount by the pumping-amountadjusting unit 13 is repeated until the groundwater level becomesstable, while detecting the pressure change in the inner tube 5 causedby this air flow by the pressure detecting unit 12. When the internalpressure of the inner tube 5 becomes stable, it is regarded that thegroundwater level reaches a stable state, and the adjustment of thepumping amount is stopped to continue the groundwater pumping operationat a stable pumping rate. That is, it is possible to avoid a situationthat the adjustment of the pumping amount by the pumping-amount controlunit 13 is not sufficient, and the groundwater level continuously lowersto reach the groundwater inlet 7, and perform the groundwater pumpingoperation at the stable pumping rate without interruption.

In addition, in the case of pumping a large amount of groundwater by thepumping unit 9 having high functions, the internal pressure of the innertube 5 changes by a small amount of air flowing in the inner tube 5 whenthe groundwater level lowers to reach the air inlet 11, as in the casedescribed above. The pressure change in the inner tube 5 is detected bythe pressure detecting unit 12, and the pumping-amount adjusting unit 13controls the pumping amount according to the output from the pressuredetecting unit 12. As the amount of groundwater decreases, fluctuationsof the groundwater level flowing in the inner tube from the air inlet 11become large. For this reason, the amount of air that comes in the innertube from the air inlet 11 often becomes unstable. In such a case, asshown in FIGS. 4 to 7, it is preferred to form the air inlet 11 havingthe larger aperture area at its lower part than its upper part. Sincethe inflow amount of air increases in a quadratic-function manner as thegroundwater level lowers, it is possible to more accurately detect thefluctuations of the groundwater level at the air inlet 11. The accuratedetection of an increase in pressure in the inner tube 5 provides astable pumping operation by the pumping unit 9.

By the way, in the present invention, it is possible to recycle theexisting wells having various diameters such as wells formed in ground 1by the conventional deep-well method, wells formed by the conventionalvacuum deep-well method, test wells for ground investigation, orrecharge wells, for the groundwater pumping apparatus of the presentinvention.

For example, as explained below, an already-existing well constructed bythe deep-well method can be used for the pumping apparatus of thepresent invention. That is, in this case, the already-existing well isused as the outer tube 3 of the present invention. First, mortar or thelike is injected into soils recharged in a space between the boring andthe peripheral surface of an upper part of the existing well to seal thesurrounding of the upper part of the existing well. Next, a pumping tubeis pull out, and an inner tube 5 having a diameter smaller than theexisting well and substantially the same length as the outer tube 3 isinserted in the outer tube 3. Subsequently, a pump is placed in theinner tube 5, and the top ends of the outer tube 5 and the inner tube 3are shielded by the shield member 6. Finally, a vacuum unit 8 is set.Thus, the already-existing well can be used as the outer tube 3 of thegroundwater pumping apparatus of the present invention.

FIG. 9 shows a groundwater pumping apparatus according to a fourthembodiment of the present invention. This pumping apparatus ischaracterized in that a plurality of water-passing portions 2 are formedin the axial direction of the outer tube 3. In this case, sincegroundwater can be collected at plural sites in the axial direction ofthe outer tube 3, it is possible to more efficiently perform thegroundwater collecting operation. When forming the air inlet 11 in thispumping apparatus, the air inlet 11 is formed at a position lower than atop end of the lowermost one of the water-passing portions 2. As theconfiguration of the air inlet 11, the slits 22 or the holes 23 for airvent described above may be used. In addition, it is preferred that theair inlet 11 has a larger aperture area at its lower part than its upperpart.

FIG. 10 shows a groundwater pumping apparatus according to a fifthembodiment of the present invention. This pumping apparatus ischaracterized by further comprising a pressure unit 14 of increasing theinternal pressure of the inner tube 5, and a water injection unit 15 forcarrying out a pressure injection of water in the inner tube 5. In thisembodiment, a vacuum pump 26 used as the vacuum unit 8 is connected to aconnection tube 27, and a compressor used as the pressure unit 14 isconnected to the connection tube 27 through a switch valve 21. The waterinjection unit 15 composed of a pressure pump is connected to the innertube 5. The pressure unit 14 and the water injection unit can be used,as described below. The operation of the vacuum unit 8 is stopped, andthen the switch valve 21 is switched to increase the internal pressureof the inner tube 5 by the pressure unit 14. In addition, the operationof the pump unit 9 is stopped, and the pressure injection of water inthe inner tube 5 is carried out by the water injection unit 15. Thereby,it is possible to clean clogging in filter members 16 around the outertube 3 and at the water-passing portion 2. Thus, when the cleaningoperation for the filter members is finished, and then the switch valveis switched to perform the groundwater pumping operation, as describedabove, it is possible to smoothly collect groundwater and efficientlypump groundwater. Therefore, it is preferred to alternately repeat thepumping operation and the cleaning operation. The pressure unit 14 andthe water injection unit 15 explained in this embodiment are availablefor the groundwater pumping apparatuses of the other embodimentsdescribed above.

1. A groundwater pumping apparatus comprising: an outer tube buried inground, which has a water-passing portion at its lower part; an innertube having a substantially same length as said outer tube and placed insaid outer tube such that said inner tube is spaced from said outertube, said inner tube having a groundwater inlet at its bottom end,which is provided at a position lower than a top end of saidwater-passing portion; a shield member configured to shield top ends ofsaid outer tube and said inner tube; a vacuum unit configured to reducea pressure in said inner tube; a pump unit configured to pumpgroundwater that comes in said inner tube through said groundwaterinlet; a pressure unit configured to increase the pressure in said innertube; and a water injection unit configured to carry out a pressureinjection of water in said inner tube.
 2. The groundwater pumpingapparatus as set forth in claim 1, wherein said outer tube has aplurality of water-passing portions formed in an axial direction of saidouter tube.
 3. The groundwater pumping apparatus as set forth in claim1, wherein said groundwater inlet is formed in a bottom end surface ofsaid inner tube and placed at a position slightly higher than the bottomend of said outer tube, and said inner tube has an air inlet composed ofat least one slit for air vent having an elongate shape, and said slitis formed in a side surface of the bottom end of said inner tube so asto extend in an axial direction of said inner tube, and said air inletis formed to extend from said groundwater inlet to a position lower thanthe top end of said water-passing portion.
 4. The groundwater pumpingapparatus as set forth in claim 3, wherein an aperture amount of a lowerpart of said air inlet is larger than the aperture amount of its upperpart.
 5. The groundwater pumping apparatus as set forth in claim 4,further comprising a pressure detecting unit configured to detect thepressure in said inner tube, and a pumping-amount adjusting unitconfigured to adjust a pumping amount.
 6. The groundwater pumpingapparatus as set forth in claim 1, wherein said groundwater inlet isformed in a bottom end surface of said inner tube and placed at aposition slightly higher than the bottom end of said outer tube, andsaid inner tube has an air inlet composed of a plurality of holes forair vent, and said holes are formed in a side surface of the bottom endof said inner tube so as to be arranged in an axial direction of saidinner tube, and said air inlet is formed to extend from said groundwaterinlet to a position lower than the top end of said water-passingportion.
 7. The groundwater pumping apparatus as set forth in claim 6,wherein an aperture amount of a lower part of said air inlet is largerthan the aperture amount of its upper part.
 8. The groundwater pumpingapparatus as set forth in claim 7, further comprising a pressuredetecting unit configured to detect the pressure in said inner tube, anda pumping-amount adjusting unit configured to adjust a pumping amount.